Diplomats say airdrops to besieged areas is a last resort, accusing government of obstructing aid delivery by land.

The United Nations has said it will ask permission from the Syrian government on Sunday to airdrop or airlift humanitarian aid to besieged areas.

During a closed-door meeting of the Security Council on Friday, diplomats described airdrops as a "last resort" to reach thousands of civilians in need of aid.

Nearly 600,000 people are besieged in 19 different areas in Syria, according to the UN, with two-thirds trapped by government forces and the rest besieged by armed opposition groups and the Islamic State of Iraq and the Levant (ISIL, or ISIS) group.

UN humanitarian chief Stephen O'Brien said that out of 34 requests for June to deliver aid to besieged and hard-to-reach areas by land convoys, the Syrian government had turned down five.

"We continue to insist that we have absolutely, as a matter of law, a need to get to those people without hindrance," he said.

The US, UK and France have long been calling for air operations, given the reluctance of Damascus to allow relief into rebel-held areas.

Syria announced on Thursday that it gave the UN and the Red Cross approval to send humanitarian aid convoys into at least 11 of the 19 besieged areas during June, as a response to the call for humanitarian airdrops.

But, several Western diplomats said the Syrian announcement is simply a ploy to deflect discussions on airdrops, noting that President Bashar al-Assad's government did not agree to permit full access to all besieged areas.

"A very high number of humanitarian access requests made by the UN have been denied by the Syrian authorities," the French ambassador to the UN, Francois Delattre, told reporters after the Security Council.

"For the month of June Syrian authorities did not accept all the access requests made by the UN.

"So, on Sunday, the UN, in accordance with the ICRC's request, will ask Damascus to authorise humanitarian airdrops to reach localities for which the land access was denied by the Syrian regime," he said. "And of course we call for the complete lifting of all sieges."

Matthew Rycroft, the UK's ambassador to the UN, said the Syrian government has done "too little too late" regarding the humanitarian crisis in the country and the international community will no longer tolerate stalling tactics.

"Airdrops are complex, costly, risky, but we have now all agreed that they are the last resort and we must use them to relieve the human suffering in so many besieged areas in Syria," he said.

Bashar Jaafari, Syria's ambassador to the UN, rejected accusations that the government was preventing aid deliveries.

"Humanitarian assistance or the humanitarian aid has never been denied by the Syrian government to any part of the country."

Syrian opposition activists have circulated a list of the UN aid that has gone into rebel-held Daraya, a town besieged by government forces. They said the aid included mosquito nets, lice shampoo, wheel chairs and a small number of medical and nutritional packages for infants.

The head of the Media Council in Daraya, Hosan Ahmad, said more cars were guarding and protecting the aid convoy than were actually delivering supplies.

He said people felt angry, humiliated and let down by the UN.

"These are luxury goods, not basic necessities for people that are desperate and eating grass."

The UN said the aid convoy that reached Daraya earlier this week was part one of a two-part delivery and the second part, which has food on it, is being delayed by Damascus.

Bays said he does not expect Damascus to change its attitude towards humanitarian aid to besieged areas, just because the UN is now talking about aid delivery by air.

"Towards the end of Bashar Jaafari's speech, I asked him repeatedly, 'yes or no, are you going to give permission?' He didn’t answer the question."

As diplomats met at the UN on Friday, volunteer rescuers said Syrian government air strikes killed dozens of civilians in and around the northern city of Aleppo.